As international standards for image coding,
(1) MPEG (Moving Picture Expert Group) for moving images (refer to “MPEG” edited by The Institute of Image Information and Television Engineers, Ohmsha, April 1996, for example);
(2) JPEG 2000 for still images (refer to “ISO/IEC 15444-1 JPEG2000 Part I: Core coding system, 2000-12-15”, for example) are well known.
MPEG is a technique adopting motion compensation and DCT (discrete cosine transform) and realizes highly efficient encoding by efficiently removing frame correlation and correlation between frames. On the other hand, JPEG 2000 is a technique that uses wavelet transform and embedded type entropy encoding called EBCOT. Although coding efficiency of JPEG 2000 is inferior to that of MPEG since JPEG 2000 does not use correlation between frames, JPEG 2000 includes various effective functions such as spatial SNR (Signal-to-Noise Ratio) scalability and the like that are not included in MPEG. Motion JPEG 2000 that can be applied to moving images is also proposed, which Motion JPEG 2000 includes the same functions of JPEG 2000.
The scalability used in JPEG 2000 is called an embedded type, in which an encoder encodes data only once and does not need to regenerate compressed data for each resolution. Decoded images of various resolutions and SNRs can be obtained from one compressed file, so that the file size and computational complexity can be decreased. FIG. 1 shows the resolution scalability function of JPEG 2000. When the resolution of the original image is K×L, the decoder can decode the image into an image of any resolution of K/2n×L/2n.
By using the above-mentioned JPEG 2000, an image that has a smaller resolution than that of the original image can be restored. However, the resolution is limited to 1/2n times (n is a positive integer) the original image. There is a problem in that, generally, the resolution of the image required in the decoding side is not limited to 1/2n times the resolution of the original image.